It is a common practice to minimize the emission of gasoline vapors from an automotive fuel tank by collecting the vapors in a canister containing charcoal, and to purge the canister by admitting its fuel vapors to the engine for combustion therein. The vapors are fed to the throttle body of the engine fuel system by a tube containing a solenoid valve for control of the purge operation. Typically purge has been allowed when the throttle is open, and engine coolant temperature, engine speed and vehicle speed are all above prescribed thresholds. The purge is controlled by a duty cycle operation of the solenoid valve. A low duty cycle is implemented when the purge is first enabled and the duty cycle gradually ramps up to 100% operation. Usually this allows the extra fuel to be accommodated by the engine and the fuel control system compensates by decreasing the injected fuel to maintain air/fuel stoichiometry. Still there are some conditions when the purge action provides too much fuel to the engine and an undesirable engine response such as surging may occur. Such excess fuel also can lead to undesirable emissions as well as to a high catalytic converter temperature.
In the fuel control system manifold pressure and temperature sensors provide information upon which a fuel amount is predicated. An exhaust oxygen sensor provides an air/fuel ratio signal to a closed loop control which generates a closed loop correction factor for adjustment of the fuel amount. When the closed loop correction factor reaches a set limit, a portion of that factor is rolled into a block learn correction factor which is stored in permanent memory and is also used for adjustment of the fuel amount. Several block learn factors are stored for use with different engine conditions, and each is updated according to the closed loop correction factor. The correction factors can compensate for purged fuel within limits. When those limits are reached it is then desirable to reduce the purge duty cycle and, consequently, the purged fuel vapor to an amount that the fuel control can manage. On the other hand it is important to maximize the purged fuel vapor rate to assure removal of the canister contents.